Depends on the region. Often there are smaller regional companies providing fiber internet. Prices for these fiber connections a still somewhat higher than the cheapest vodafone tier, but you also get better service for your money.

Afaik almost a monopoly: there is Deutsche Telekom which does the same thing and Vodafone. I think apart from some local providers almost everybody else is just a reseller of one of the two.

This is a bit messy but if there are completing options at a given location install multiple ISPs and run them concurrently and log the details—download speeds, etc.

There's nothing as good as hard verifiable data—even if regulators play hardball and favor ISPs then you've the evidence to whip up political action (claim biased decisions, etc.).

There's no monopoly, it is a cartel. Most big businesses in Germany operate in cartels to fuck the customer.

In Germany in specific building there is only one provider available in your telephone socket, and one in your cable socket in your apartment. Frequently there is no cable socket.

Sounds like their largest competitor (DT) is already doing this.

No monopoly. Only for cable internet, which may be a possible argument. For landline internet (DSL), there's plenty of alternatives.

Yes and no. There are other providers in Germany. However, with the EU's neoliberal privatization policy the governments privatized many existing infrastructure. Vodafone bought the previous government company that owned all of the the cable TV infrastructure of Germany. So they are a monopoly of a particular type of internet connection. Depending on the place the alternatives could be too slow since Germany also has an aging population that do not {care about, demand} higher internet speeds and didn't upgrade its copper infrastructure due to corruption.